DIGESTION 205 



exposure to the absorbing mucosa. There is some evidence also that the 

 anti-peristaltic waves may force some of the liquefied contents through the 

 ileo-colic opening into the small intestine because of the relaxation of the ilio- 

 colic sphincter muscle. It is questionable if this ascending movement is a 

 true peristalsis inasmuch as the advancing contraction is apparently not pre- 

 ceded by an area of inhibition or relaxation. It resembles rather the corre- 

 sponding movement manifested by the small intestine to which the term 

 anastalsis has been given, and is propagated along the muscle coat independ- 

 ently of the myenteric plexus. 



In addition to this anastaltic wave, haustral contractions have been ob- 

 served which resemble the segmentation contractions of the small intestines 

 and which promote, it is believed, the absorption or drawing of water from 

 the intestinal contents. 



The contents of the large intestine are thus deprived by absorption of 

 the excess of water and of any possible nutritive material. The remainder 

 of the material is then gradually transferred by peristaltic waves down the 

 descending colon to the sigmoid flexure where it accumulates prior to its 

 expulsion from the body. 



The Function of the Large Intestine. The large intestine, by reason 

 of its anatomic relation to the small intestine, serves as a receptacle for the 

 temporary storage of the indigestible residue of the food together with certain 

 excretions of the intestinal glands both of which have descended from the 

 small intestine. Inasmuch as the contents of the large intestine, in that 

 portion known as the ascending colon are quite liquid, while the contents 

 in the portion known as the descending colon are more or less solid, it is 

 apparent that an absorption of liquid must take place during the transit 

 from the cecum to the rectum. This is made possible by the retardation of 

 the intestinal contents caused by the antiperistaltic or the anastaltic wave, 

 together with the haustral contractions. Between the two modes of activity 

 different portions of the intestinal contents are exposed to the mucosa and 

 by which the liquids are in large measure absorbed. In the distal portion 

 of the large intestine, the characteristic movement is peristaltic by which the 

 more solid contents are carried to the sigmoid flexure where they accumulate 

 until the desire to evacuate the mass is experienced. By the contraction of 

 the pelvic portion of the descending colon the mass is forced into the rectum 

 from which it is discharged from the body. The function of the large 

 intestine is therefore to receive, to reduce to a proper consistency, to tempor- 

 arily store and subsequently discharge its contents, consisting of the indi- 

 gestible residue of the food, together with excretions of intestinal glands 

 which have descended from the small intestine and which constitute in part 

 the feces. 



The Nerve Mechanism of the Large Intestine. The nerve mechan- 

 ism of the large intestine includes both motor and inhibitor nerves. The 

 motor nerves comprise both pre- and post-ganglionic fibers; the former 

 have their origin in the spinal cord, from which they emerge in the third and 

 fourth sacral nerves and pass by way of the pelvic nerve to the pelvic 

 ganglia around the cells of which their fibers arborize; the latter (post- 

 ganglionic) fibers emerge from the cells of these ganglia and are distributed 

 to circular and longitudinal muscle-fibers of the intestinal wall. 



The inhibitor fibers also comprise both pre- and post-ganglionic fibers; 



